Mine is set to "E" by default, I guess. I have my main (OEM) "C" partition, followed by "D", my recovery partition, then "E", my CD, then "F", my second Windows 7 partition (evaluation OS).
Then when I boot into the other partition, it's altogether different. It claims itself the "C" partition. I just let it fall where it may.
But as far as the topic goes, these days, a complete scan with your choice of AV software on a weekly basis is a necessity. And at least every other week, use another product such as MBAM 1.50 or ESET Online Scanner. Backup drives, as well as camera cards (if you insert it into your computer) should also be scanned once every few months. Flash drives that you frequently use should be scanned whenever you scan your computer, as infections can easily reside there, oftentimes in an exe. file.
If you can't use your browser, and you know your internet signal is OK, scan immediately without delay. Or if you notice system changes that you didn't make.
I realize that this is a thing that is often overlooked, but it's maintenance, just as like when you change your oil in your ride, it's done to prevent problems, at regular intervals. The threat of bad code is at an all time high, and doing your part to prevent it is your best shot at having a clean and smooth running computer.